Add placetype counts by country chart to website

[stepps00]

A placeholder issue for after whosonfirst/whosonfirst-cookbook#40 is merged.

The placetype counts per country chart is useful, and it'd be nice to include on the whosonfirst.org website. Similar to how we maintain a "key terms" section on the Documentation page, we could add a new section that would link to the table.

Something like:

[nvkelso]

Thanks! I'd love to see this new column item next to the existing placetypes row.

Would also be great to link to the stats / counts doc from the general documentation for placetypes, too.